Abstract

Background and Purpose: Stroke occurs mostly in patients with advanced age and older patients have a less favorable prognosis than younger patients. To understand the underlying mechanisms involved in this phenomenon, we tested the hypothesis that an increased inflammatory response to acute ischemic injury in the aged mice leads to more severe brain damage and functional deficits.

Methods: An ischemic stroke model was created in two- (young) and twelve-month-old (aged) C57BL/6 mice (n=6) through permanent occlusion of the left distal middle cerebral artery (pMCAO). Infarct volumes in the brain sections were quantified at one day after pMCAO by Cresyl Violet staining. Sensorimotor function was assessed using corner test and adhesive removal test before, 3, and 14 days after pMCAO. Macrophage/microglia (CD68+ cells) and apoptotic neurons (TUNEL+ and NeuN+) in the ischemic region were quantified one day after pMCAO. The interleukin-6 (IL-6) and interleukin-1 β (IL-1β) levels in the ischemic brain tissue were measured using ELISA at one and three days after pMCAO.

Results: At one day post pMCAO, infarct volume was larger in aged mice than in young mice (31±3/mm3 vs 27±5, p= 0.05), Comparing aged mice to young mice, more TUNEL+ neurons (22±5 vs 15±5, p=0.04) were detected in the infarct region. Both young and aged mice showed significant sensorimotor dysfunction on day 3 and 14 post pMCAO; turning more frequently to the left on the corner test and taking longer time to remove tape on left paws. More severe functional deficits were detected on aged mice by adhesive removal test at day 14 post pMCAO (12±5 seconds vs 6±3, p=0.01, aged vs young). There were more CD68+ cells in the peri-infarct region in aged mice compared to young mice (79±22/ 40X objective field vs 58±18, p=0.01). Infarct volume was positively correlated with both the numbers of CD68+ cells (R2= 0.80) and TUNEL+ neurons (R2= 0.86). Compared to young, cytokine levels also increased in aged mice at one day (IL-6: 308 ±152 pg/mg vs 147±44, p=0.03; IL-1β: 96±35 vs 55±8, p=0.02) and three days (IL-6: 149 ±53 vs 90±27, p=0.03; IL-1β: 55 ±14 vs 38± 5, p= 0.02) after pMCAO.

Conclusion: Our data suggest that a higher inflammatory response at the acute stage of ischemic stroke in aged mice is associated with more severe neuronal injury (larger infarct volume and more apoptotic neurons) and long-term functional deficits. The upstream mechanisms responsible for the enhanced response will require additional studies. However, modulating inflammatory response in the aged population at the acute stage might be a strategy to reduce neuronal injury and improve functional recovery in older stroke patients with monitoring of brain inflammation a feasible means of risk stratification.